Field of the Invention
This invention relates to an improved dosed loop single mixed refrigerant process wherein an improved efficiency is accomplished by the use of a cooling and liquid refrigerant separation step between a first and second stage compressor in combination with reconstitution of the mixed refrigerant prior to use of the compressed mixed refrigerant.
In recent years, the demand for natural gas has increased. In many instances, natural gas is found in areas which are remotely located from the markets for the natural gas. Unless the natural gas is located sufficiently close to a market place so that it is feasible to construct a pipeline to transport the natural gas, it must be transported by tankers or the like. The transportation of natural gas as a gas requires prohibitively large tanker volumes; therefore, the natural gas is customarily liquefied for storage and transportation. The use of liquefied natural gas is well known and methods for its storage and use are well known. Natural gas may also be liquefied at the point of use when it is available in surplus but may be needed in larger volumes than can be delivered to the point of use in the future and the like. Such storage may be used, for instance, to meet a wintertime peak demand for natural gas in excess of that available through an existing pipeline system during the winter peak demand periods or the like. Various other industrial applications require that natural gas be liquefied for storage and the like.
Other gases are liquefied with somewhat less frequency but may also be liquefied by the improved process described herein.
Previously, substances such as natural gas have been liquefied by processes such as shown in U.S. Pat. No.4,033,735 issued Jul. 5, 1977 to Leonard K. Swenson which is hereby incorporated in its entirety by reference. In such processes, a single mixed refrigerant is used. Such processes have many advantages over other processes such as cascade systems, in that they require less expensive equipment and are less difficult to control than cascade type processes. Unfortunately, the single mixed refrigerant processes require somewhat more power than the cascade systems.
Cascade systems such as the system shown in U.S. Pat. No. 3,855,810 issued Dec. 24, 1974 to Simon, et al. basically utilize a plurality of refrigeration zones in which refrigerants of decreasing boiling points are vaporized to produce a coolant. In such systems, the highest boiling refrigerant, alone or with other refrigerants, is typically compressed, condensed and separated for cooling in a first refrigeration zone. The compressed cooled highest boiling point refrigerant is then flashed to provide a cold refrigeration stream which is used to cool the compressed highest boiling refrigerant in the first refrigeration zone. In the first refrigeration zone, some of the lower boiling refrigerants may also be cooled and subsequently condensed and passed to vaporization to function as a coolant in a second or subsequent refrigeration zone and the like. As a result, the compression is primarily of the highest boiling refrigerant and is somewhat more efficient than when the entire single mixed refrigerant stream must be compressed.
In view of the reduced equipment cost and reduced difficulty of control with the single mixed refrigerant process, a search has been directed to the development of such a process wherein the power requirements are reduced.